Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2002-05-31
2004-03-30
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S181800, C162S168300, C162S183000
Reexamination Certificate
active
06712934
ABSTRACT:
The present invention relates to a method for the production of paper and board, wherein there is used as a retention aid in the retention system a solution of a cationic polymer together with a microparticle mixture which contains a swellable clay of the smectite group.
At present, the use of microparticles in the retention system of paper production, in particular in the production of fine paper, is very common, the aim being to improve further the efficiency of the production process. The advantages of the adoption into use of microparticles include improved retention, more efficient dewatering, and better formation. The most effective of the microparticles in use are colloidal silica-based microparticles of various types, solid or sol, and bentonite-like swellable natural materials belonging to the smectite group of clays. Instead of, or in addition to, a microparticulate compound it is possible to use as a retention aid in the retention system polymers, which may be anionic, cationic or non-ionic, and which are characterized by a high molecular weight. The problem involved with these compounds is typically excessive flocculation, which deteriorates the optical properties of paper.
The silicates may be natural crystalline minerals or synthetic materials. Synthetic silicates have the advantage of better controllable properties, in which case the efficiency of the microparticulate material used can be maximized. The colloidal synthetic silicates used as retention aids in retention systems include, for example, colloidal silica and polysilicate, aluminum silicates, and aluminum silicates modified with alkali metals and with alkaline-earth metals. The particle size of these materials is typically a few nanometers or a few tens of nanometers, and they are more expensive than, for example, bentonite.
The minerals of the smectite group of natural clays include montmorillonite, beidellite, nontronite, saponite and sauconite, which are composed mainly of aluminum silicates and some of which contain, in addition to sodium, also other cations, such as magnesium, iron, calcium or zinc. Smectites also include hectorite and vermiculite, which are, instead, composed mainly of magnesium silicate and contain to a lesser extent also other cations. Natural clays are typically somewhat darker than synthetic materials, owing to impurities present in them.
Bentonite is a species of rock mainly composed of montmorillonite (Kirk-Othmer Encyclopedia of Chemical Technology, Part 6, 4
th
edition, p. 394). However, the name bentonite is commonly also used of commercial products which contain mainly montmorillonite. Bentonite-type materials have been used in paper production especially as materials adsorbing impurities. Natural hectorite is mainly composed of magnesium silicate. In hectorite, some of the exchangeable sodium ions have been replaced by lithium ions. In addition the structure contains some fluoride.
Bentonite has been used as a retention aid in paper production together with a cationic polymer in the patent U.S. Pat. No. 4,753,710 of Allied Colloids. In the process according to the patent, a cationic polymer, preferably polyethylene imine, a polyamine epichlorohydrin product, a polymer of diallyl dimethyl arrmonium chloride, or a polymer of acrylic monomers, was added to an aqueous cellulosic suspension before the last shearing stage, and bentonite was added after this shearing stage. Improved retention, dewatering, drying, and web forming properties were thereby achieved. In the microparticle system according to the method there is used bentonite, which is available under the trade name HYDROCOL.
Respectively, in the paper production method according to the patent U.S. Pat. No. 5,178,730 of Delta Chemicals, there is added to the pulp before the shearing stage a cationic polymer, which is preferably a tertiary or quaternary amine derivative of polyacrylamide, and after the shearing stage, before the headbox, there is added a natural hectorite at a weight ratio of 0.5:1-10:1. It has been observed that the combination of polymer and hectorite used in the method affects filler retention and dewatering more effectively than does, for example, bentonite used in a corresponding manner. The method according to the patent can be used in both alkaline and acid paper production recipes.
In the patent U.S. Pat. No. 5,876,563 of Allied Colloids, a cationic starch together with a cationic polymer and an anionic microparticulate material is used as the retention aid. The microparticulate material suggested for use in this connection is, for example, bentonite or colloidal silica or polysilicate microgels or polysilicic acid microgels together with aluminum-modified colloidal silica, or aluminum-modified polysilicate mnicrogel or aluminum-modified polysilicic acid microgel, of which a suspension is formed.
In the application WO 99/14432 of Allied Colloids, the microparticulate aid is preferably bentonite, colloidal silica, polysilicic acid, polysilicate microgel, or an aluminum-modified version thereof.
In Finnish patents 67735 and 67736, there is used, together with a hydrophobic size, a retention aid combination which contains, together with a polymer, preferably polyacrylamide, as an anionic component a colloidal silicic acid, bentonite, carboxymethyl cellulose or carboxylated polyacrylamide.
The use of silicate microparticles together with a cationic polymer in a retention system is described in the patent U.S. Pat. No. 5,194,120 of Delta Chemicals. The prevalent cation in the synthetic amorphous metal silicate was Mg, and the polymer was preferably a ternary or quaternary amine derivative of polyacrylamide, their weight ratio being between 0.03:1 and 30:1. By the method, retention, dewatering and formation were improved by using smaller amounts of retention aids than previously, and thus the costs were correspondingly lower.
According to our observations, when bentonite is used together with polyacrylamide, it serves as an effective microparticulate material in the retention system. Compared with this, a synthetic metal silicate in which the prevalent cation is Mg is, in a corresponding situation, not as effective as bentonite.
We have observed, surprisingly, that when there is used a microparticle mixture in which the major part consists of bentonite or hectorite and to which a small amount of a synthetic metal silicate having magnesium as the prevalent cation is added, the said mixture serves as a microparticulate material more effectively than does either component of the mixture, bentonite or hectorite or synthetic metal silicate, separately.
According to the invention there is thus provided a method for producing paper or board in such a manner that retention aids are added to the stock stream passing to the paper machine headbox, the stock stream is directed to the wire, the stock is dewatered in order to form a paper web, and the paper web is dried, the method being characterized in that the retention aids used are a solution of a water-soluble cationic polymer and a microparticle mixture which contains, in the form of a suspension, a swellable clay of the smectite group and a colloidal synthetic metal silicate, the prevalent cation in the synthetic metal silicate in the suspension being magnesium.
The said swellable clay of the smectite group, hereinafter in the specification referred to as clay material, is preferably bentonite or hectorite.
The microparticle mixture in the form of a suspension is preferably prepared by mixing the said clay material, preferably bentonite or hectorite, and the said metal silicate together while dry. A suspension is made from the dry mixture by slurrying the dry mixture in water, preferably to a concentration of 1-20%, and especially preferably to a concentration of approx. 5%.
The microparticle mixture can be transported and stored in the form of a suspension, but preferably the microparticle mixture is transported and stored in a dry form, and a suspension is prepared from it on site, immediately before use.
The proportion of the clay material in the mi
Ahlgren Jonni
Strengell Kimmo
Birch & Stewart Kolasch & Birch, LLP
Chin Peter
Hug Eric
Kemira Chemicals Oy
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